Problem: Simplify the following expression: $a = \dfrac{1}{9n} + \dfrac{2}{3n}$
Answer: In order to add expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $9n$ and $3n$ $\lcm(9n, 3n) = 9n$ $ a = \dfrac{1}{1} \cdot \dfrac{1}{9n} + \dfrac{3}{3} \cdot \dfrac{2}{3n} $ $a = \dfrac{1}{9n} + \dfrac{6}{9n}$ $a = \dfrac{1 +6}{9n}$ $a = \dfrac{7}{9n}$